1. Field of the Invention
This invention relates to moisture-curing jointing compounds based on NCO-terminated polyurethane prepolymers. More particularly, the invention relates to moisture-curing jointing compounds having improved properties through the addition of reactive polysiloxanes.
2. Discussion of Related Art
Moisture-curing polyurethane-based sealing compounds are well known to the expert, for example from DE 37 26 547. They consist essentially of isocyanate-terminated polyurethane prepolymers and swollen polymers present therein as thixotropic agent or filler. In addition, plasticizers, swelling aids, pigments, dyes, stabilizers, catalysts and other auxiliaries are mentioned as further constituents in DE 37 26 547. Although known polyurethane jointing compounds largely meet the stringent demands made of them, some aspects are still in need of improvement. This applies in particular to weathering resistance. A key factor in this regard is UV stability. A key criterion for determining the long-term stability of weathered jointing compounds, apart from visual evaluation, are the elongation at break values which the present invention seeks to improve. Improvements are also desirable in regard to the processing properties, such as stringing and smoothability.
It is known to the expert, for example from DE-OS 32 21 655, that UV absorbers, such as benzophenone or trioxazole derivatives, can be added to sealing compounds to improve their UV stability. These absorbers provide increased, but not permanent, UV stability. It is known to the expert that jointing compounds based on polysiloxanes normally show higher UV stability than comparable polyurethane sealing compounds. However, commercially available polysiloxane sealing compounds generally have undesirable properties for certain applications, including for example poor spreadability, "greasiness" and odor emission during processing. Mixtures of the two components can only be produced with considerable effort, if at all, and are entirely unsatisfactory in regard to storability, in other words the systems in question have a pronounced tendency to separate. Attempts to counteract this tendency by additives, such as dispersion aids and/or emulsifiers, result in unwanted deteriorations in other performance of the jointing compounds.
The use of reactive polysiloxanes in polyurethane sealing compounds to improve their weathering stability has not hitherto been known to the expert. Although compounds of siloxanes and urethanes are known in principle, these compounds differ from the present invention in their molecular structure and/or are used in a different technological field. Thus, EP 250 248 describes block copolymers of polysiloxanes and urethane segments which are produced from diaminopolysiloxanes and diisocyanates and which are used as pressure-sensitive (contact) adhesives. According to European patent application 293 084, reactive polysiloxanes are present in polyurethane gloss paints to improve their anti-friction properties.
The problem addressed by the present invention was to provide moisture-curing polyurethane jointing compounds which would show improved long-term stability under weathering conditions and, in particular, to UV radiation. In addition, the moisture-curing polyurethane jointing compounds according to the invention would show distinctly improved processing properties, such as stringing and smoothability.